Denigrate wild mares? (5)
I believe the answer is:
smear
'denigrate' is the definition.
('smear' can be a synonym of 'denigrate')
'wild mares?' is the wordplay.
'wild' indicates an anagram.
'mares' with letters rearranged gives 'SMEAR'.
(Other definitions for smear that I've seen before include "Medical examiner may take this" , "Discredit" , "Slanderous denigration of character" , "Sully" , "Calumniate" .)